Plant nutrients can be divided into three main classes:                Primary or macronutrients: nitrogen (N), phosphorus (P) and potassium (K).        Secondary nutrients: calcium (Ca), magnesium (Mg), sulphur (S), sodium (Na).        Micronutrients: boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn).        
Particulate solid forms of inorganic fertiliser such as granules or prills represent the most common type of fertiliser used in agriculture, incorporating at least the primary or macronutrients (the so-called NPK-fertilizers), and often secondary nutrients. Particulate solid fertilisers are commonly applied to the soil in order to provide the growing crop with the bulk of its requirement for primary and secondary nutrients.
There is often a requirement to also include micronutrients in particulate solid fertiliser products in order to meet the agronomic requirements of the crop. This can be achieved by incorporating micronutrients during the prilling or granulation process. Alternatively, WO 9915480 (Norsk Hydro, 1999) describes how micronutrients can be coated onto particulate fertilisers by application of an aqueous solution of an acid and a mineral base. However, practical considerations in high volume production operations imply that it is difficult to satisfy the widely different nutrient requirements of different crops and different soil types using any of the aforementioned approaches. Furthermore, the use of an aqueous solution on an hygroscopic material such as calcium ammonium nitrate and ammonium nitrate fertilizer is not recommended.
Physical blending of powdered or granular micronutrient components with solid fertilisers offers more flexibility in terms of manufacture but the end product suffers from several disadvantages. The differences in particle size and density between the different components can lead to segregation during storage and handling which can result in uneven application to the soil and crop. Another disadvantage, particularly where powdered micronutrients are used, is the dusting that can occur during transfer and application. This not only leads to uneven application but also presents a potential environmental or health and safety risk.
WO 03071855 (Ade & Company, 2003) teaches a method by which a fertiliser can be coated with micronutrient applied in the form of a fine, dry powder which is claimed to produce a low dusting product. However, if the original fertiliser substrate is itself naturally dusty, this method offers no possibility of reducing the inherent dustiness.
These problems can be reduced to a certain extent by applying an oil, surfactant or binding polymer during the blending process, e.g. by spraying, but this adds a further step to the process and thereby increases the complexity of the blending operation.
Micronutrients may be added to solid fertiliser by coating the granules using an aqueous slurry or suspension of the micronutrient components. This method can result in a low dusting product with an even distribution of micronutrient. However, the introduction of water using this technique (albeit a very small the amount) can compromise the storage stability of the solid fertiliser by increasing its tendency to cake or by reducing the strength of the prills or granules, especially with nitrate based solid hygroscopic materials such as calcium ammonium nitrate and ammonium nitrate, and with urea. Furthermore, it is easy to make a mistake by adding more of the aqueous slurry or suspension to the solid fertilizer than necessary, which has a detrimental effect on the whole batch in terms of swelling and caking tendency.
It has been proposed in U.S. Pat. No. 3,692,529 (Rychman, 1972) to treat solid carrier particles with an adhering oil, and a pigment or colouring compound. Subsequently, the thus oiled carrier particles are blended with granular solids to provide a free-flowing non-segregating, homogeneous composition. The function of the oiled carrier particles is to provide free-flowing properties to the granular solids composition which would otherwise be non-free flowing. The granular solids are liable to give off dust, and may require further processing steps to suppress dusting.
Other approaches have required special apparatus to add components separately and create chemical or chelation reactions in situ, in particular between the particulate solid fertilizer material and the components, and/or the use of elevated temperatures. Examples are: US 2005/076687, CN102603431, WO 2011/080764, U.S. Pat. No. 4,657,576, GB 954,555 and CN 102358710.
Hence, there is a need for a better method to post-treat solid fertilizer particles to at least party overcome the problems from the prior art.